The invention relates to stable fluid systems to be used in combination with a solid oxidizer in preparing an explosive composition. More particularly, the invention relates to a microemulsion that may be used to provide an explosive composition with a greater density than a typical nitrate/fuel oil explosive.
Mixtures of ammonium nitrate (AN) and diesel fuel oil (FO) have been used for many years in the explosives industry. Typically, ammonium nitrate in prill form is mixed with diesel fuel oil in the ratio of about 94to 6, and the mixture has come to be known as ANFO. ANFO is inexpensive and is widely used in various kinds of blasting, but its relatively low bulk density (about 0.8 g/cc) limits the amount of useful energy that can be obtained per borehole. ANFO also becomes desensitized by water which precludes its sse in water-filled boreholes.
Various attempts have been made to increase the density or bulk strength of ANFO, and thereby provide more energy per volume. Some examples of these attempts include the use of high density additive fuels (e.g. ferrophosphorous), crushing the ammonium nitrate, and the use of thickened water-based AN slurries. The use of high density fuels requires special equipment for addition of the fuels to the prills which increases the cost of the explosive. Similarly, special equipment and personnel are required for partially crushing the prills which also results in increased costs. Slurries have the problem of lacking sensitivity and require the addition of sensitizing agents as well as additional equipment.
U.S. Pat. No. 3,764,421 describes another attempt to solve the density problem of ANFO that includes adding water in controlled amounts to a prilled ANFO, aging the resulting mixture for a period of time (typically 10-14 days), and ther mixing the prilled ANFO such that it breaks down into finely-divided solids. This approach essentially achieves the same result as partially crushing the prills but uses aging instead of special equipment. A need still exists for a method and formulation for increasing the density of a solid oxidizer based system over that obtainable with ANFO without the use of special equipment or aging.
The explosives art has also sought to improve the sensitivity of ANFO in various ways. Australian Pat. No. 281537 to Coxon describes an explosive using ammonium nitrate prills with an emulsion of fuel oil, water and an anionic surface agent or emulsifier. Coxon attempted to improve the sensitivity of ANFO by adding a small amount of water and distributing it with oil in the form of an emulsion over the ammonium nitrate In this manner, Coxon achieved greater intimacy between the oil and the AN thereby achieving greater sensitivity. Coxon describes oil-in-water emulsions in which water is the continuous phase as being generally more stable, and therefore, preferred over water-in-oil emulsions. For Coxon's intended use, the emulsion need only be stable for a few minutes after mixing.
The explosives industry addressed the problem of making a waterproof explosive using ammonium nitrate and fuel oil by forming the components into a water gel or a water-in-oil emulsion. U.S. Pat. No. 3,447,978 to Bluhm discloses a water-in-oil emulsion explosive in which an aqueous solution of oxidizing salts form the discontinuous aqueous phase, and the continuous phase is formed with a fuel. The emulsion also has an occluded gas component and an emulsifier. The occluded gas was included to lower the density of the emulsion thereby increasing the sensitivity. Without the occluded gas, the emulsion is not detonable. Later patents, such as U.S. Pat. No. 3,765,964 included sensitizers such as strontimm in addition to occluded gas to increase the sensitivity of the emulsion.
Numerous other patents also describe explosive compositions that incorporate oxidizing agents as part of the aqueous phase of an emulsion. Examples include U.S. Pat. No. 3,161,551 to Egly et al. which discloses a water-in-oil emulsion containing 50-70% by weight of ammonium nitrate, 15-35% water, 5-20% of an organic sensitizer and a small amount of emulsifiers that may be combined with particulate ammonium nitrate. Egly teaches to combine the emulsion with particulate ammonium nitrate so as to fill all the spaces between the particles. U.S. Pat. No. 3,356,547 to Berthmann et al. describes an emulsion containing nitroglycerin that is used with solid AN particles.
Clay in U.S. Pat. No. 4,111,727 discloses an explosive composition formed by mixing 10 to 40% of a water-in-oil emulsion containing an oxidizer salt dissolved in the water phase with 60 to 90% of solid oxidizer such as ammonium nitrate. The two components are mixed such that sufficient air is left in the interstitial spaces of the solid oxidizer to render the mixture detonable. The emulsion does not need to contain occluded gas.
Clay in U.S. Pat. No. 4,181,546 discloses a waterproof explosive comprising 40 to 60% by weight of a solid, particulate oxidizer salt and 60 to 40% of a water-in-oil emulsion containing an oxidizer salt dissolved in the water and combined with an oil component held in a stable emulsion condition by a small quantity of emulsifier. The emulsion also contains a density controlled sensitizer such as hollow glass beads, polystyrene beads, microballoons or the equivalent. The components are thoroughly mixed together to substantially eliminate voids between the solid granules.
In a later patent, U.S. Pat. No. 4,294,633, Clay disclosed a blasting composition of 60 to 90% by weight of solid particulate oxidizer salt and 10 to 40% of a liquid slurry partially filling the interstices and voids between the solid particles. The slurry is a substantially saturated and thickened solution of strong oxiiizer salt so as not to appreciably dissolve or soften the granules.
A disadvantage of water-in-oil emulsions in which the aqueous phase contains dissolved oxidizer salts is that the emulsions are highly viscous compared to diesel fuel oil and require special handling and equipment. Also, such emulsions are relatively unstable and will separate or "break" into different phases on temperature cycling. When such emulsions are used in mixtures as described in the Clay U.S. Pat. Nos. 4,181,546 and 4,111,727 patents, they are generally stored separately until mixed with the solid oxidizer particles. In order to prevent phase separation in cold climates, it is usually necessary to heat the emulsion continuously from production until use. These same disadvantages are characteristic of almost all of the emulsions presently used in the explosives industry. They all exhibit limited stability over time and sensitivity to temperature cycling.
U.S. Pat. No. 4,555,278, to Cescon, et al. describes a stable blend of nitrate particles and a water-in-oil emulsion formed with an anionic emulsifying agent comprising a fatty acid salt. The stability of the blend is achieved by controlling the cell size of the dispersed aqueous phase in the emulsion so as to decrease the chemical driving force between the water and the solid oxidizer. Cescon states that "the optimum cell size of the internal phase of an emulsion in a blend is the largest that will not crystallize on losing water over the goal shelf life of the product." (Col. 7 lines 46-48). Cescon further recites that the optimum cell size is within the range 1-4 microns, "decreasing as the aqueous phase water content decreases." (Col. 7 lines 52-53).
Another example of an explosive emulsifier system is disclosed in U.S. Pat. No. 4,357,184 to Binet. Binet discloses explosive systems consisting of synthetic polymeric emulsifiers that produce a relatively stable water-in-oil emulsion. The emulsions comprise an aqueous solution of at least one oxygen-supplying salt as a discontinuous phase, an insoluble liquid or liquefiable carbonaceous fuel as a continuous phase, a sensitizing component and a blend of emulsifying agents. Binet describes the emulsions as "ultra-stable colloidal dispersions" and uses the term microemulsion. As used by Binet, the term microemulsion describes a liquid-liquid foam of very small cell size ranging from about 1 micron to about 15 microns. In the emulsion art, however, the term microemulsion means something different than that described by Binet. What Binet termed a microemulsion is more properly termed a small cell macroemulsion.
Contrary to the use in Binet, the term "microemulsion" as used in the emulsion art, and as used in describing the present invention, is a system of water, oil and amphiphile(s) which spontaneously form a liquid solution with droplets or cells of less than 0.1 microns in diameter. Macroemulsions are generally recognized as having a cell size greater than 1 micron as disclosed in Binet and Cescon. "Amphiphile(s)" are surfactant and cosurfactant species. Microemulsions are generally recognized as being thermodynamically stable, i.e., infinitely stable over a fixed range of temperatures and pressures. Thermodynamic stability also implies that the emulsions form spontaneously without the input of additional energy. Macroemulsions, on the other hand, are inherently unstable and are useful for only a limited time. Extreme conditions in transport, storage and handling may significantly reduce the useful life of a macroemulsion. Another characteristic of macroemulsions is that they require energy to form, e.g. usually vigorous mixing. Special equipment is necessary to accomplish this mixing. In its lowest energy state, the microemulsion will form essentially a single, homogeneous phase with small microdroplets. By contrast, a macroemulsion is a two-phase system. Generally, microemulsions are optically isotropic which implies that a beam of polarized light will be refracted through the solution in the same way regardless of the angle of the beam, although anisotropy is recognized in some microemulsions. Macroemulsions are usually opaque and sometimes translucent.
The fluid systems of th present invention exhibit the characteristics of a true microemulsion. In particular, the fluid systems exhibit remarkable stability that allows for extended storage and use under varying conditions. In addition, when the fluid systems are added to a solid oxidizer they act to increase the density of the solid and of the resulting explosive system. These features result in a very desirable explosive composition. Indeed, the explosive compositions of the present invention can be used as a replacement for ANFO while using the same equipment as is presently used for ANFO and providing a product with a greater density and bulk strength.